1. Field of the Invention
The field of the invention relates generally to variable displacement engines for providing improved fuel economy.
2. Background of the Invention
Various methods are available for air-fuel ratio control. In some systems, as the driver adjusts the throttle, fuel injection is adjusted to maintain a desired air-fuel ratio. In other systems, airflow is adjusted via an electronic throttle to match injected fuel.
The inventors have recognized a problem. In particular, if fuel is controlled to control engine output, then during the stoichiometric mode fuel cannot be used to control air-fuel ratio since it is constrained to control engine output. Alternatively, if fuel is controlled to control the air-fuel ratio, then during lean operation fuel cannot be used to control engine output since it is constrained by air-fuel ratio.
Also, the inventors herein have proposed operating some cylinders with substantially no injected fuel, while operating other cylinders at various air-fuel ratios. This increases the lean combustion limit (since the engine is operating at a higher load) and reduces engine pumping work. However, if an exhaust air-fuel ratio sensor sees a mixture air (from the cylinders without injected fuel) and combusted gasses (from operating cylinders), then the sensor reading does not match the combustion air-fuel ratio.
Various of the above problems are solved by controlling an engine having first and second groups of combustion chambers and an engine exhaust, operating the engine in a first operating mode where (i) air and substantially no injected fuel are provided to the first combustion chamber group, (ii) both air and injected fuel are provided to the second combustion chamber group at a lean air-fuel ratio, and (iii) injected fuel is adjusted to the second combustion chamber group. The engine can also be operated in a second mode where both air and injected fuel are provided to both the first and second combustion chamber groups and at least one of the air and the injected fuel provided to both the first and second combustion groups is adjusted to control the air-fuel ratio of the first and second groups.
In particular, torque is controlled with fuel when some cylinders operate with substantially no injected fuel, while air-fuel ratio can be controlled by, for example, airflow. In other operating modes, air-fuel ratio can be controlled by injecting fuel to all of the cylinders.
In another aspect of the present invention, the method comprises reading a sensor output of a sensor exposed to a mixture of air and substantially no injected fuel in a first stream from the engine and a combusted lean mixture of air and fuel in a second stream from the engine, and adjusting a measured air-fuel ratio of the combusted lean air and fuel mixture to account for the air in the first stream. The method further comprises treating the combusted air and fuel in a catalyst coupled to the engine that retains oxidants including NOX when operating lean and reduces the retained oxidants when operating rich.
Here, the engine control corrects for the pure airflow that would otherwise cause the air-fuel sensor to give an incorrect indication of the combustion air-fuel ratio. Note, in the specific case of a V-8 engine where 4 cylinders operate with injected fuel and 4 cylinders operate with substantially no injected fuel, the desired air/fuel would be double to account for the pure air being seen by the sensor. Alternatively, the measured air-fuel from the sensor could be halved.
In still another aspect of the present invention, engine air-fuel ratio is estimated based on catalyst temperature. In particular, the method comprises operating the engine in a mode where air and injected fuel are provided to the first combustion chamber group and air and substantially no injected fuel are provided to the second combustion chamber group. The method further comprises adjusting one of the air and the injected fuel provided to the first combustion chamber group to adjust the air-fuel ratio of the first group to change between lean and rich, measuring the temperature of the emission control device and calculating the air-fuel ratio of the first cylinder group based on the temperature.
In particular, when the engine is lean, there are virtually no reductants to react exothermically across the catalyst with excess oxygen, and therefore catalyst temperature indicates a value that would be expected. However, if the combusting cylinders operate slightly rich of stoichiometry, the rich gasses (reductants) can react exothermically across the catalyst with excess oxygen (from the cylinders that have substantially no injected fuel) and thereby raise the temperature of the catalyst beyond that expected. As such, the method can indicate when the engine transitions between lean and rich, and therefore can give an estimate of engine air-fuel ratio.